Characterization of the amsI gene product as a low molecular weight acid phosphatase controlling exopolysaccharide synthesis of Erwinia amylovora

The ams region, responsible for amylovoran synthesis of the fireblight pathogen Erwinia amylovora, contains the gene amsI encoding a 144 amino acid protein with homology to mammalian low molecular weight acid phosphatases [Bugert and Geider (1995) Mol. Microbiol. 15, 917-933]. A DNA fragment with amsI was cloned under the control of the lac promoter on a high copy number plasmid. The gene product of amsI is about 17 kDa in a protein expression system and had the enzymatic activity of an acid phosphatase. This is the first report about a low molecular weight acid phosphatase activity in prokaryotes. As part of the large ams transcript, expression of amsI was affected by the activator proteins RcsA and RcsB. Over-expression of amsI in E. amylovora caused a strong increase of acid phosphatase activity, but additionally a strong reduction in EPS synthesis, phenotypically similar to a mutation in the gene. The gene product may participate in changes of phosphorylation required for the biosynthesis of EPS such as recycling the Lipid carrier diphosphate to the monophosphate form.